1. Field of the Invention
Illustrative embodiments of the present invention relate to a parameter generation sheet for generating a parameter to correct image tone in a digital copier, a facsimile machine, a printer, or any other type of apparatus capable of forming a gradation image, an image processing device using the sheet, an image processing method using the sheet, and a storage medium storing program codes causing a computer to execute the method using the sheet.
2. Description of the Background
Image output apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional peripherals having two or more of the foregoing capabilities. When such an image output apparatus outputs images using the same image data over time, the output images may come to have different image densities. To correct such variation in image density over time, a set of test patches may be output from the image output apparatus and read with, e.g., a scanner to generate parameters to correct image tone.
However, even if images are output using the same image data, the output images have different densities at different areas. In a conventional electrophotographic-type image output apparatus, such variation in image density may be attributable, for example, to problems in the quality or assembly accuracy of components, such as eccentricity in the rotation of a photoconductor or a transfer roller or variation in a gap between the photoconductor and a development sleeve in a direction along a rotation shaft of the photoconductor.
Hence, certain conventional image forming apparatuses have been proposed that form a plurality of test patches having an identical density (tone) value, determine the densities of the patches, and calculate an average of the densities to generate density-correction parameters to correct variation in image density over time while correcting localized differences in image density.
One conventional printer uses different types of screens depending on the type of target object. For example, a lower LPI (lines-per-inch) screen is used for picture-and-pattern objects. By contrast, a higher LPI screen or a substantially-random screen through error diffusion processing is used for character or line objects.
Alternatively, each target area of a document may be identified as an edge portion or a non-edge portion, with a lower LPI screen used for the non-edge portion and a higher LPI screen or a substantially-random screen through error diffusion processing used for character or line portions.
In other words, the lower LPI screen capable of stably expressing image gradation is used for a picture-and-pattern portion or a non-edge portion in which tone expression is relatively important, and the higher LPI screen capable of more accurately expressing details is used for a character or line object in which resolution and sharpness are relatively more important than gradation. In this regard, since different types of screens may have different relations between tone values and densities, it may be preferable to output tone correction patches for each screen type to generate tone correction parameters.
As described above, in generating tone correction parameters, one conventional technique outputs a plurality of test patches of an identical color and tone in a parameter generation sheet to generate tone correction parameters, and calculates an average of detected densities of the patches to correct localized differences in image density. However, outputting a plurality of patches for each screen type increases the number of output patches, circuit size, calculation time, consumed amounts of toner, ink, and other consumables. Further, such an increased number of patches necessitates an increased print area, resulting in an increased number of print pages.